The invention relates to a method for operating a diesel engine, which involves switching between a lean mode with superstoichiometric combustion air ratio λ>1 and a rich mode with substoichiometric combustion air ratio λ<1.
Devices for adsorbing nitrogen oxide (NOx) are used to clean the exhaust gas of diesel engines. Under certain conditions, e.g. available storage capacity, active temperature window, NOx adsorber systems store the nitrogen oxides from internal combustion engines during lean combustion, i.e. superstoichiometric combustion with λ>1 and residual oxygen in the exhaust gas. NOx adsorber systems also store the sulfur which is present in the fuel and engine oil in the form of sulfates (SOx). On account of the higher chemical bonding forces, the sulfates undesirably occupy the storage areas for the nitrogen oxides. To regenerate NOx adsorber systems of this type, i.e. to desorb NOx and, at the same time, convert NOx and to desorb SOx and at the same time convert SOx, oxygen-free exhaust gas with λ<1 and the highest possible reducing-agent content is required. Regeneration of an NOx adsorber system of this nature can be realized by temporarily rich engine operation, i.e. with substoichiometric combustion with λ<1.
Patent DE 195 43 219 C1 describes a method for operating a diesel engine which involves switching between a lean mode with superstoichiometric combustion air ration λ>1 and a rich mode with substoichiometric combustion air ratio λ<1. The switching is carried out in order to regenerate an exhaust-gas cleaning installation with an NOx adsorber system. The switching from lean mode to rich mode is effected by electronically controlled exhaust gas recirculation, intake air throttling, additional injection of fuel and an increase in the exhaust gas back pressure.
Patent DE 197 50 226 C1 describes an engine control system for a diesel engine in which the switching between lean mode and rich mode is simultaneously accompanied by switching between stored characteristic diagrams for the lean mode and characteristic diagrams for the rich mode. The intention of this method, inter alia, is that the driver should not be disturbed by, or notice, the switching between lean mode and rich mode. However, the diesel engine only supplies approximately the same output as in the corresponding lean mode within a limited range of the rich mode, and consequently an unnoticed change between the operating modes can only take place within this limited range.
Patent DE 197 53 718 C1 describes a method for operating a diesel engine in which the diesel engine is only switched from lean mode to rich mode when there is a steady-state or quasi-steady-state engine operating mode present. The intention of this is that the driver should not notice any change in the power provided by the diesel engine during the switching. If there are non-steady-state engine operating conditions in rich mode, the engine is switched back from rich mode to lean mode. Both characteristic diagrams for lean mode and for rich mode of the diesel engine are stored in a memory in the engine management system.
DE 196 36 790 A1 describes the setting of a Diesel engine to produce a rich exhaust-gas mixture. In such a way power losses as a result of the rich exhaust-gas mixture are prevented in that the rich mixture is set only under a low load, under engine overrun conditions or during idling.
In DE 196 36 040 A1, it is proposed to achieve a rich exhaust-gas mixture for the regeneration of an NOx storage device by increasing an exhaust gas recirculation rate. In the event of relatively high loads on the internal combustion engine, in particular ≧20% of the rated output, it is proposed to reduce the ratio of recirculated exhaust gas to intake air in order to counteract a power drop.
DE 199 14 787 A1 describes an exhaust-gas cleaning system for a diesel engine, in which a reducing agent is injected into the exhaust line in order to regenerate an NOx storage device and the exhaust-gas flow quantity is reduced using an exhaust-gas throttle valve. Since a regeneration mode of this type increases the pump losses of the engine, reduces the engine output and therefore causes a sudden change in torque, the quantity of fuel injected into the combustion chamber and the degree of opening of an exhaust-gas recirculation valve are increased, in order to ensure the same engine power as before the reduction in the exhaust-gas flow quantity.
It is the object of the invention to provide a method for operating a diesel engine and a diesel engine in which it is possible to switch between a lean mode and a rich mode of the diesel engine without the driver noticing, a switch over and without losses of driving comfort.